The invention concerns a method for automated measurement of the ohmic rotor resistance of an asynchronous machine, which is controlled via an inverter, while being acted upon by a non-rotating field.
In an asynchronous motor, whose speed and torque are controlled, particularly according to a field oriented control method, knowledge of all resistances, that is the ohmic and inductive resistances, is required to make the control as accurate as possible. They can be assessed and/or measured.
Measurements are made either with rotatable, unloaded rotor or with blocked (braked) rotor. When a testing current for measuring the resistances is led through the stator at unloaded rotor, the larger share of the current will flow through the main reactance, which is determined by the main inductance (counter-inductance), thus enabling a measurement of the main inductance, but not of the ohmic rotor resistance. When the measurement is made with blocked rotor, however, the testing current also flows through the rotor, so that also its ohmic resistance can be measured. Both methods, however, involve disadvantages.
A measurement with rotating unloaded rotor is often not possible, for example when the motor is fixedly incorporated in a finished product, and its axis is fixedly loaded. On the other hand a blocking of the motor, particularly when full torque is applied, places heavy demands on the mechanical braking device, so that this method is substantially more expensive. Another difficulty in connection with measurements on a blocked rotor is the current displacement in the rotor bars occurring at high frequencies in the range from 30 to 60 Hz, causing too high a measurement value of the ohmic rotor resistance.
Further, on measuring the ohmic resistance, its variations in dependence of the operating temperature are often not considered. Depending on the operating temperature, it can increase or decrease by 20% to 30%. This means that the equivalent diagram of the asynchronous machine forming the basis of the measurement does not apply for the normal operation.
U.S. Pat. No. 5,689,169 shows a method, in which the stator and rotor leakage inductances and the ohmic rotor resistance at stillstanding rotor are measured by controlling the q-components and the d-components in a xe2x80x9cfield orientedxe2x80x9d control process. Thus, one phase winding of the stator receives a testing signal with a frequency, which is approximately equal to the operating frequency, and, for example, amounts to 30 Hz. The current component Iq is set at zero to avoid production of a rotating torque, and at the same time the actual voltages Vq and Vd fed back to the control device are measured. With known testing signal frequency and previously measured ohmic stator resistance, the approximate value of the rotor resistance can be calculated. The reason for this approximation is that the testing signal frequency is chosen to be relatively high, so that relatively simple mathematic equations can be used for the calculation, demanding only little calculation performance from a microprocessor used in the control device. However, the relatively high testing signal frequency of approximately 30 Hz has the disadvantage that a current displacement takes place in the rotor bars, which results in a too high measurement value of the ohmic rotor resistance. In extreme cases the measurement value can be 100% to 150% too high. This method, compared with a converter with an inverter having only current sensors, has the additional disadvantage that also voltage sensors must be used.
A method of estimating equivalent circuit parameter values of an induction motor is known from a paper in EPE""97 by Godbersen et al., Danfoss Drives A/S, Denmark, pages 3.370 to 3.374. According to this method a number of measurements and calculations are made and evaluated in accordance with a conventional equivalent diagram (FIG. 1 of the paper) of an asynchronous motor. That equivalent diagram is substantially as shown in FIG. 2 of the drawings of the present application. By the method presented in the paper, all the desired parameter values of the motor except the ohmic rotor resistance Rr will be obtained. The paper does not describe in detail how to calculate the ohmic rotor resistance from the parameters obtained. Furthermore the applicants have found that, even though it is possible in principle to calculate the ohmic rotor resistance from the parameter values obtained with the method presented in the paper, the resistance value as actually calculated from those parameter values has too large an error.
The invention is based on the task of determining the rotor resistance of an asynchronous machine faster than hitherto, and at the same time preventing measurement faults caused by a current displacement.
The solution of this task according to the invention comprises a method for automated measurement of the ohmic rotor resistance of an asynchronous machine controlled via an inverter while being acted upon by a non-rotating field, the method involving
a) measuring the ohmic stator resistance, the leakage inductances and the main inductance of the asynchronous machine,
b) applying a testing signal formed by a predetermined direct signal with a superimposed alternating signal to a phase winding of the asynchronous machine, the frequency of the alternating signal corresponding approximately to the nominal slip frequency of the asynchronous machine,
c) measuring the amplitude and the phase of the phase signal resulting from the test signal, and d)calculating the ohmic rotor resistance according to the measured values of a) and c).
With this solution, one measurement of the resulting phase signal in dependence of the testing signal will be sufficient. Accordingly, the measuring duration is reduced. As the frequency of the alternating signal corresponds approximately to the very low nominal slip frequency of the asynchronous machine, with which the asynchronous machine runs during operation and which results from the known frequency of the rotating field and the nominal speed of the asynchronous machine and is relatively low, also measurement inaccuracies caused by a current displacement disappear. The DC-value of the testing signal in b) above is used to bring the main inductance to a predetermined magnetizing level. The alternating signal, which is used to generate a phase displacement between the voltage of the testing signal and a measured phase current enables the calculation of the referred rotor resistance, and has a frequency which must be carefully chosen. On the one hand, if this frequency is too high, current displacement in the rotor bars will occur resulting in an erroneous value of the rotor resistance. On the other hand, choosing a frequency too low causes the current to flow through the main inductance instead of the rotor resistance.
Preferably, the ohmic rotor resistance referred to the stator side is determined first, and the actual ohmic rotor resistance is calculated by means of the measurement values according to a) and c).
Preferably, the frequency of the alternating signal is in the range from 1 to 8 Hz.
Advantageously, the direct signal is a direct voltage, which is chosen so that the resulting direct current is less than half the nominal magnetising current of the asynchronous machine. The nominal magnetising current is the current that is needed to magnetise the asynchronous machine to the level where it develops rated power.
Advantageously, the direct current amperage is chosen so that the dynamic main inductance is approximately equal to the static main inductance of the asynchronous machine.
It may be provided that the testing signal is a phase voltage, whose reference value is set on the basis of a previously measured characteristic, stored in a memory, displaying the dependency of the phase current on the reference value.